The above described method makes use of the effect that an evanescence field is formed on the surface of a waveguide when electromagnetic radiation is moved through a waveguide (by total reflection), which gives off no energy to the surroundings when no perturbations are present. But if a perturbation in the form of an absorbing medium is present in this zone, which has a thickness of not more than approximately one wavelength of the electromagnetic radiation coupled in, it can be coupled to the evanescent field and extract energy from it. By waveguide is meant here the part carrying the electromagnetic radiation. For example, if an optical core/cladding fiber is used, the core of the fiber is to be understood as the waveguide in the above sense.
The physical phenomenon is used in evanescence field spectroscopy, also known as ATR (“attenuated total reflection”) technology, for the detection of specific radiation-absorbing substances in fluids. This technology can also be used, for example, in the spectroscopy of strongly absorbing media, such as lubricants containing soot particles, especially oils, which are not suitable for the classical absorption spectroscopy on account of too intense and broad-band attenuation of the radiation and therefore less useful signal dynamics, or at least cannot produce any reliable findings. Sample applications for evanescence field spectroscopy are given in the publications DE 102 51 893 A1, WO 2004/113886 A1 or EP 0594838 B1.
Starting from this known method, measures have been discussed in many scientific publications for boosting the sensitivity of evanescence field spectroscopy. Merely as examples, reference is made to the works of Kudlaty, et al., Development of an infrared sensor for on-line analysis of lubricant deterioration, Sensors, Proceedings of IEEE (2003), 2, 903; Mizaikoff, Mid-infrared evanescent wave sensor—a novel approach for subsea monitoring, Meas. Sci. Technol. 10 (1999), 1185; Villatoro et al., In-line optical fiber sensors based on cladded multimode optical fibers, Appl. Opt. 43 (2004), 5933 or Sheeba, Fibre optic sensors for the detection of adulterant traces in coconut oil, Mess. Sci. Technol. 16 (2005), 2247.
In the publications US 2006/0228257 A1 and U.S. Pat. No. 6,899,849 B2, methods of evanescent field measurement on biological specimens are discussed, which are supported by means of electric fields that cause an electroosmotic or dielectrophoretic flux in the medium being studied.
All of the known proposal are addressed to specific applications and therefore have the same specific drawbacks in other application instances. For example, one can mention a lack of suitability for analysis of dispersions or emulsions, expensive techniques for functionalizing the waveguide, lack of long-term stability of the corresponding coatings, slow response times and poor irreversibility of diffusion-controlled processes and quite generally expensive manufacturing methods and often steep instrumentation requirements which must be considered unfit for industrial applications or measurement uses from a financial standpoint.